Multimode optical fibers through each of which a plurality of modes propagate include step index (SI) fibers and graded index (GI) fibers. SI multimode optical fibers each include a core made of, e.g., pure quartz, and each have a uniform refractive index profile. By contrast, GI multimode optical fibers each include a core made of, e.g., quartz doped with germanium (Ge), and each have a refractive index profile called a graded profile along the diameter of the core. While SI multimode optical fibers have the problem of mode dispersion in which the propagation delay time increases with increasing mode order, GI multimode optical fibers are designed to have a graded profile in order to provide uniform propagation delay times of modes, thereby reducing the mode dispersion to a low level. Such graded profiles generally include a square distribution profile.
PATENT DOCUMENT 1 describes an optical transmission fiber including a GI core having a square refractive index distribution profile, a first cladding surrounding the core and having a thickness of 1/100- 1/20 of the core diameter, a second cladding surrounding the first cladding and having a lower refractive index than the first cladding, and a third cladding surrounding the second cladding and having a lower refractive index than the second cladding.
PATENT DOCUMENT 2 describes a GI multimode optical fiber including a core having an α-th power refractive index profile and a cladding surrounding the core, and configured so that an outer peripheral portion of the cladding forms a depressed region. Such a depressed region has a lower refractive index than an inner peripheral portion of the cladding, and has a multiple-step-like refractive index profile in which the refractive index increases toward the outer periphery of the cladding. PATENT DOCUMENT 2 further describes that such a configuration can provide excellent bending loss characteristics of a GI multimode optical fiber.